1. Field of the Invention
Granulators are employed in the plastics industry in particular to reduce the size of the plastic particles to facilitate the removal of contaminants therefrom during the production of recycled resin such as, for example, polyethylene terephthalate PET.
2. Description of the Prior Art
Typically, the recycling process involves a two-stage process, although the process can be carried out in a single stage. The first stage involves the grinding of the material to be recycled to a nominal size of three-eighths inch (xe2x85x9cxe2x80x3). This is accomplished by introducing the material to be recycled into a chamber of the granulator which is provided with rotating rows of knives that continue to cut the material being processed until the material is small enough to exit the first through holes in a screen in the bottom of the first stage of the process. The diameter of the holes in the screen is xe2x85x9cxe2x80x3.
It has been found difficult to reduce the diameter of the flake being produced by merely reducing the size of the holes in the exit screen. Generally, the requirement for a smaller particle size has resulted in a reduction in the size of the holes in the exit screen, as well as an increase in the rotational speed of the cutting mechanism. These efforts have caused a number of production problems including an increased production of dust and fines which cannot be recycled. The dust and fines tend to clog the exit screens and require increased operating energy to operate the cutting mechanism. The increased usage of energy results in a build-up of heat energy which must be dissipated in order to avoid damage to the attendant equipment. It will be understood that a number of factors come into play when grinding plastic material to a small particle size. Traditional grinding requires the weight of the flake (material being treated), airflow through the granulator, and an external air moving system under the exit screen to perform a push-pull operation to get the material through the holes in the screen as quickly as possible. In the event the transit time of the material being processed increases, the material remains in the cutting chamber for longer periods of time resulting in the material being continuously cut into smaller and smaller pieces. The longer it takes for the material to transit through the cutting chamber and the exit screen, the lower the production rate, increased wear and tear on the cutting knives, and more dust and fines (unusable product) are produced. Conventional granulators and grinders and associated screens are not capable of achieving improved results because the smaller the particles become, the less pliable they become. These less pliable particles are not able to readily pass through the small holes in the exit screens.
It is an object of the present invention to produce a granulator capable of producing flake of a diameter of four (4) millimeters or less to facilitate the decontamination of the material by removal of contaminants therefrom.
Another object of the invention is to produce a granulator capable of producing small diameter flake at production rates superior to those of the prior art.
Another object of the invention is to produce a granulator for comminuted plastic material in the recycling process whereas the wear and tear in the associated cutting knives is greatly minimized.
Still another object of the invention is to produce a granulator wherein there is reduced stress on the cutting mechanism and associated exit screen.
Another object of the invention is to produce a granulator having a reduced quantity of dust and fines.
The above, as well as other objects of the invention may be surprisingly achieved in a granulator including rotary knives, associated bed knives, a rotor for carrying the rotary knives for sequential shearing action between the rotary knives and the bed knives, an exit screen allowing the passage of comminuted material less than four (4) millimeters diameter, and an air handling system creating a pressure drop across the screen to facilitate the passage of the processed material therethrough, wherein the screen is provided with a plurality of spaced apart holes.